x86Bootstrap
x86 MBR bootloader wroten in GNU assembly syntax.
Some really good explaination...
Multiboot handled by a menu from which you can boot on the 4 MBR partitions.
Dependencies
You can compile the only source file with your hands if make is missing.
-
make(dev under V4.3) -
as(dev under V2.34) -
ld(dev under V2.34)
Build
make
Install
dd if=bootstrap.bin of=/dev/sdX bs=446 count=1
Master Boot Record Format
| Offset | Size (bytes) | Description |
|---|---|---|
| 0x000 | 440 | MBR Bootstrap (flat binary executable code) |
| 0x1B8 | 4 | Optional "Unique Disk ID / Signature"2 |
| 0x1BC | 2 | Optional, reserved 0x00003 |
| 0x1BE | 16 | First partition table entry |
| 0x1CE | 16 | Second partition table entry |
| 0x1DE | 16 | Third partition table entry |
| 0x1EE | 16 | Fourth partition table entry |
| 0x1FE | 2 | (0x55, 0xAA) "Valid bootsector" signature bytes |
Traditional MBR steps
- relocate itself to 0x0000:0x0600
- examine the byte at offset 0x1be, 0x1ce, 0x1de, and 0x1ee to determine the active partition
- load only the first sector of the active partition (which is expected to contain a DOS bootsector) to 0x0000:0x7c00 (hence the previous relocation)
- set SI
- jump to 0x7c00 -- transferring control to the DOS bootsector.
Partition Table Entry
| Offset1 | Size (bytes) | Description |
|---|---|---|
| 0x00 | 1 | Drive attributes (bit 7 set = active or bootable) |
| 0x01 | 3 | CHS Address of partition start |
| 0x04 | 1 | Partition type |
| 0x05 | 3 | CHS address of last partition sector |
| 0x08 | 4 | LBA of partition start |
| 0x0C | 4 | Number of sectors in partition |
Runtime
Epilogue
Just for the fun.
Feel free to fork, use, improve.